Electroencephalographic measures of the neurophysiological dysfunction underlying autism have been nonspecific and incomplete. Studies using electroencephalographic methods have been fraught with subject sampling bias, a lack of standardized techniques and measures, and a lack of appropriate control groups. Low-functioning autistic children with age-matched normals, age-matched mentally handicapped, and mentally age-matched normal toddlers were tested using a computerized electroencephalographic technique. The autistic children showed significantly more slow wave activity and less alpha, as well as less inter- and intrahemispheric asymmetry than either normal or mentally handicapped children. In general, electroencephalographic features of autistic children closely resembled those of the toddlers, supporting a model of maturational lag as the key descriptor for autistic CNS functioning. A model of diminished cortical differentiation is proposed to account for the low level of intellectual functioning.